It should be changed though. He should be able to send another app to the kingdom. Only this time the leader should get a message stating that this player has already been in his kingdom and got kicked/left for some reason. Then the leader can decide whether to take the player in again or not.

I see your reasoning, and mistakes happen, and its not fair. But most generally, that player was kicked for a good reason, and being kicked from a KD should not be a simple decision, if it is, then their should be no reason at all for them to rejoin.